The present invention relates, in general, to micromachined capacitance type accelerometers, and more particularly to annular mass accelerometers using electrostatic forces to position the annular mass.
Accelerometers are generally based on the principle of a seismic mass suspended in some way by a spring system, with acceleration being measured indirectly by measuring displacement of the seismic mass from a neutral position. Sensitivity of the accelerometer, that is to say the displacement for a given acceleration, depends on two quantities: the size of the seismic mass and the strength of the spring. The seismic mass can easily be made to remain stable indefinitely but the strength (modulus of elasticity) of the spring may vary, changing the displacement of the seismic mass for a given acceleration and making the accelerometer lose accuracy or in extreme cases making the accelerometer unserviceable.
There are numerous factors which can cause a spring to change its characteristics, a few of these are: fatigue of the material due to flexure, deformation due to being kept under stress, changes in strength and size due to temperature changes, non-linear strength changes as displacement of the mass increases, and deformation or breakage by transient stresses from a shock. These factors may be minimized by the correct selection of spring material and by careful design of the spring itself, but they are significant to some degree in every apparatus that depends on a spring for its operation. Eliminating the shortcomings of the spring would greatly enhance the design of an accelerometer in just about every way.
Micromachined accelerometers have made it possible to reduce the effect of the spring by using more easily manipulated electrostatic forces instead. Previous approaches have used a single block as a seismic mass; however it is extremely hard to control twisting effects with an electrostatic control system using this geometry for the seismic mass. As a result, the mechanical spring strength must still be a significant factor in operation of the accelerometer with the electrostatic forces being used primarily for a damping effect to reduce excessive motion of the seismic mass.